The instant invention includes improved, large area photovoltaic devices formed from a plurality of electrically interconnected smaller area photovoltaic cells, as well as methods for the manufacture of such devices. The large area photovoltaic device of the instant invention has a larger active area available for the conversion of incident light to electricity as compared to previously available photovoltaic devices because masking of the active are a by the current carrying bus bars of the device is eliminated.
Single crystal photovoltaic devices, especially silicon photovoltaic devices, have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent, and consume no expendable natural resources in their operation. However, the utility of such devices is limited by problems associated with the manufacture thereof. More particularly, single crystal materials (1) are difficult to produce in sizes substantially larger than several inches in diameter, (2) are thicker and heavier than their thin film counterparts; and (3) are expensive and time consuming to fabricate.
Recently, considerable efforts have been made to develop processes for depositing amorphous semiconductor films, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n type devices substantially equivalent to those produced by their crystalline counterparts. It is to be noted that the term "amorphous" as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions. As used herein, the term "microcrystalline" is defined as a unique class of said amorphous material characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as band gap, electrical conductivity, and absorption constant occurs. It is to be noted that pursuant to the foregoing definitions a microcrystalline semiconductor alloy falls within the generic term "amorphous".
For many years, such work with amorphous silicon or germanium films was substantially unproductive because of the presence therein of microvoids and dangling bonds which produce a high density of localized states in the energy gap, which states are derogatory to the electrical properties of such films. Initially, the reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein silane (SiH.sub.4) gas is passed through a reaction tube, where the gas is decomposed by a radio frequency (r.f.) glow discharge and deposited on a substrate maintained at a temperature of about 500-600 degrees K. (227-327 degrees C.). The material so deposited on the substrate is an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material an N-dopant such as phosphine gas (PH.sub.3), or a P-dopant such as diborane (B.sub.2 H.sub.6) gas, is premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The material so deposited includes supposedly substitutional phosphorus or boron dopants and is shown to be extrinsic and of n or p conduction type. The hydrogen in the silane was found to combine, at an optimum temperature, with many of the dangling bonds of the silicon during the glow discharge deposition to substantially reduce the density of the localized states in the energy gap, thereby causing the amorphous material to more nearly approximate the corresponding crystalline material.
It is now possible to prepare by low discharge or vapor deposition thin film amorphous silicon or germanium alloys in large areas, said alloys possessing acceptable concentrations of localized states in the energy gaps thereof and high quality electronic properties. Suitable techniques are fully described in U.S. Pat. No. 4,226,898, entitled "Amorphous Semiconductor Equivalent to Crystalline Semiconductors," of Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980, in U.S. Pat. No. 4,217,374, under the same title to Stanford R. Ovshinky and Masatsugu Izu, which issued on Aug. 12, 1980, U.S. Pat. No. 4,504,518 of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens, entitled "Method of Making Amorphous Semiconductor Alloys and Devices Using Microwave Energy," which patents are assigned to the assignees of the instant invention, the disclosures of which are incorporated herein by reference. As disclosed in these patents, it is believed that compensating agents such as fluorine and/or hydrogen introduced into the amorphous semiconductor operate to substantially reduce the density of the localized states therein and facilitate the addition of other alloying materials.
Since amorphous alloys may be readily deposited atop a wide variety of substrates and over large areas, it is now possible to readily fabricate multiple cell stacked photovoltaic structures. The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was disclosed at least as early as 1955 by E. D. Jackson, in U.S. Pat. No. 2,949,498, issued Aug. 16, 1960. The multiple cell structures therein disclosed utilized p-n junction crystalline semiconductor devices. Essentially, the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell while the short circuit current remains substantially constant. It should be noted that Jackson employed crystalline semiconductor materials for the fabrication of the stacked cell device; however, it is virtually impossible to match lattice constants of differing crystalline materials. Therefore, it is not possible to fabricate such crystalline tandem structures in a commercially feasible manner. As the assignee of the instant invention has shown, such tandem structures are not only possible, but can be economically fabricated in large areas by employing amorphous materials.
It is of obvious commercial importance to be able to mass produce photovoltaic devices such as solar cells. However, with crystalline cells, mass production was limited to batch processing techniques by the inherent growth requirements of the crystals. Unlike crystalline silicon, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in the following U.S. Pat. No. 4,400,409, for A Method of Making P-Doped Silicon Films And Devices Made Therefrom; U.S. Pat. No. 4,410,588, for Continuous Amorphous Solar Cell Deposition And Isolation System And Method; U.S. Pat. No. 4,542,711 for Continuous Systems For Depositing Amorphous Semiconductor Material; U.S. Pat. No. 4,492,181, for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells; and U.S. Pat. No. 4,485,125 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these Patents, the disclosures of which are incorporated herein by reference, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a solar cell of p-i-n type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy.
Since each deposited semiconductor alloy, and especially the intrinsic semiconductor alloy, must be of high purity: (1) the deposition environment in the intrinsic depositon chamber is isolated, by specially designed gas gates, from the doping constituents within the other chambers to prevent the diffusion of doping constituents into the intrinsic chamber; (2) the substrate is carefully cleansed prior to initiation of the deposition process to remove contaminants; (3) all of the chambers which combine to form the deposition apparatus are sealed and leak checked to prevent the influx of environmental contaminants; (4) the deposition apparatus is pumped down and flushed with a sweep gas to remove contaminants from the interior walls thereof; and (5) only the purest reaction gases are employed to form the deposited semiconductor materials. In other words, every possible precaution is taken to insure that the sanctity of the vacuum envelope formed by the various chambers of the deposition apparatus remains uncontaminated by impurities, regardless of origin.
The layers of semiconductor material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, photodiodes, phototransistors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
While various configurations of thin film photovoltaic cells may, as detailed hereinabove, be readily fabricated, it is frequently desirable to form large area modules out of cells. Incorporation of smaller area cells into large area modules offers several advantages. First, and most obviously, the surface area of the resulting module can be made quite large so as to provide for large scale power production. Secondly, by assembling a module from series and/or parallel interconnected cells, the voltage and current characteristics of the resulting module may be readily selected to match the power output thereof to specific applications. Thirdly, forming cells into modules allows for the manufacture of very high efficiency photovoltaic devices, insofar as the modules may be formed from the very best smaller area cells available.
Large area modules are generally formed by electrically interconnecting smaller area photovoltaic cells in the appropriate series and/or parallel configuration. The completed module is frequently encapsulated in a protective, light transmissive coating so as to assure mechanical integrity thereof and protection from ambient conditions. It should be noted that for purposes of clarity and explanation the module will be described herein as a large area photovoltaic device comprised of electrically interconnected smaller area photovoltaic cells. The term "smaller area photovoltaic cell" is used in its broadest sense, and is intended to designate any sub-portion of the module capable of providing an electrical current in response to the absorption of light. The term "cell" is not meant to be limited to a single n-i-p type p-n type or other photovoltaic device, but is meant to include stacked tandem arrays of photovoltaic cells. The term "smaller area photovoltaic cell" is used herein is also meant to include variously configured devices which may also include several interconnected photovoltaic regions thereupon. Generally speaking, the term "smaller area photovoltaic cell" is simply meant to define any photovoltaic unit, however configured, which is assembled with other such units to provide a large area photovoltaic device. Specifically, the term "smaller area photovoltaic cell" includes relatively larger strips which may be interconnected according to the principles disclosed herein.
It is obviously desirable to manufacture large area photovoltaic modules, as is evidenced by the variety of configurations of modules presently known. A more detailed discussion of some of such module designs will be had hereinbelow.
Clearly, it is desirable for any module to have the maximum photoconversion efficiency attainable. The efficiency may be improved via two routes: (1) the semiconductor materials, and the cells fabricated therefrom may be optimized to yield the highest photoconversion efficiency and (2) the configuration of the module itself may be optimized to secure the maximum advantage of the improved semiconductor materials and cells.
Now that the assignee of the instant invention is capable of manufacturing high quality photovoltaic alloys and incorporating those alloys into highly efficient photovoltaic cell configurations, it is desirable to maximize the efficiency of modules fabricated therefrom so as to make full use of these improved materials and cells. One significant source of loss in the efficiency of large area photovoltaic cell modules is resultant from shading of the active area thereof by portions of the current collecting bus grid system.
As will be described in greater detail hereinbelow, photovoltaic cells, particularly large area photovoltaic cells require a current collection system, generally referred to as a bus-grid system, for collecting photogenerated current and conveying that current to a collection point such as a terminal of the device. The bus-grid system is typically formed of a highly conductive material such as a metallic member, or a conductive ink or paste pattern. The various materials employed for the fabrication of the bus-grid system are optically opaque and thus shade portions of the photovoltaic cell disposed directly therebeneath, rendering those portions inactive. While it is necessary to have a bus-grid system in order to increase the collection efficiency of the cell, losses resulting from the shading are derogatory to the performance of the cell. Accordingly, a device performance has heretofore been optimized by striking a fine balance between: (1) the series resistance imposed upon the photovoltaic device by the bus-grid pattern; (2) loss of efficiency resultant from bus-grid shading. Since the bus bar portion of the bus-grid pattern is by far the member which occupies the largest area, it would therefore be desirable to have a large area photovoltaic device having no losses resultant from bus bar shading of photovoltaic material.
In accord with the principles of the instant invention, applicants have designed and manufactured an improved large area photovoltaic device which eliminates the problem of bus bar shading. The photovoltaic device of the instant invention allows for maximum utilization of the surface thereof for the photo generation of electrical current and accordingly, allows for full utilization of the photoconversion efficiency of assignee's semiconductor alloys and cells.